The present invention relates to a solid electrolyte capacitor and a method of forming the same, and more particularly to a solid electrolyte capacitor using a conductive polymer and being improved in reliability and a method of forming the same.
As the requirements for down-sizing and improvement in high speed performances and digitalization of electronic devices have increased, also it has now been required to reduce the scale of the capacitor and increase the capacitance thereof as well as reduce the impedance in high frequency band.
In prior art, mica capacitors and multilayer ceramic capacitors have been known as capacitors useable in high frequency ranges. The increase in the capacitance of those capacitors likely to result in enlargement in size of the capacitors, for which reason those capacitors are unsuitable for down-sizing. Namely, the mica capacitors and the multilayer ceramic capacitors arc advantageous in the high frequency performances.
Various electrolyte capacitors such as aluminum electrolyte capacitors and tantalum solid electrolyte capacitors have been known in prior art as being advantages in large capacitance. Those electrolyte capacitors are, however, disadvantages in high resistivity of electrolyte. For example, the aluminum electrolyte capacitor is disadvantageous in high resistivity of its electrolytic solution. The tantalum solid electrolyte capacitor is disadvantageous in high resistivity of manganese dioxide. Such high resistivity of the electrolyte makes it difficult to realize a low impedance characteristic in high frequency range.
In Japanese laid-open patent publication No. 52-79255, it is disclosed to use a low-resistive organic semiconductor, for example, 7,7,8,8-tetracyanoquinodimethane complex salt (TCNQ salt) for electrolyte of the electrolyte capacitor in order to reduce the impedance in high frequency range.
In Japanese patent publication No. 4-56445, it is disclosed to use a conductive polymer, which is lower in resistivity than manganese dioxide and TCNQ salt, for a solid electrolyte thereby to obtain a further reduction in impedance in high frequency range. This solid electrolyte capacitor using the conductive polymer has a great deal of advantages in lower resistance of the conductive polymer, for which reason this solid electrolyte capacitor is attractive and actively developed to further improve its other characteristics.
In Japanese laid-open patent publication No. 2-74021, it is disclosed that an anodic oxidation or an anodization is carried out to form an dielectric oxide coating film on a surface of a valve metal for subsequent surface treatment thereof by use of a silane coupling agent, a titanium coupling agent or an aluminum coupling agent, before an electrolytic polymerization is carried out to form a conductive polymer as a solid electrolyte. It is also disclosed that an anodic oxidation or an anodization is carried out to form an dielectric oxide coating film on a surface of a valve metal for subsequent adhesion of manganese dioxide on the dielectric oxide coating film on the surface of the valve metal and a further surface treatment thereof by use of a silane coupling agent, a titanium coupling agent or an aluminum coupling agent, before an electrolytic polymerization is carried out to form a conductive polymer as a solid electrolyte. The coupling agents described above are used to improve in adhesiveness between a porous layer and a conductive polymer layer as well as between manganese dioxide and the electrically conductive polymer film in order to prevent any substantive reduction in electrostatic capacitance and any deterioration in loss factor under a high temperature condition.
In Japanese laid-open patent publication No. 4-73924, it is disclosed that an dielectric oxide coating film is formed on a surface of a valve metal as an anodic foil for subsequent polymerization to form a conductive polymer on the anodic foil, wherein a silane coupling agent is present between the anodic foil and the conductive polymer, so as to improve in adhesiveness between the porous layer and the conductive polymer layer, thereby preventing any substantive reduction in electrostatic capacitance and any deterioration in loss factor under a high temperature condition.
In Japanese laid-open patent publication No. 8-293436, there is disclosed a solid electrolyte capacitor having a solid electrolyte comprising a conductive polymer, wherein an electron donors organic compound between an dielectric oxide coating film and a conductive polymer compound layer, wherein the electron donors organic compound is except for anion surfactant, silane coupling agent, aluminum coupling agent and titanium coupling agent. In the process for forming the electron donor organic compound layer, a porous pellet is exposed to a vapor of the electron donors organic compound, whereby a thin and uniform film of the electron donors organic compound is formed on the porous pellet independently from the kinds of the electron donors organic compounds. Alternatively, it is also possible to dip or immerse the porous pellet into a solution containing the electron donors organic compound. Those techniques provide the solid electrolyte capacitor free from any increased leakage of current under the high temperature conditions.
In Japanese laid-open patent publication No. 9-45591, there is disclosed a solid electrolyte capacitor having a semiconductor layer comprising a conductive polymer as illustrated in FIG. 1, wherein an anode lead 9 is embedded in a tantalum porous pellet 1 having a surface coated with a Ta.sub.2 O.sub.5 dielectric oxide coating film 2 which is further coated with a conductive polymer film 4 which is furthermore coated with a graphite layer 5 which is more over coated with a silver paste layer 6. Further, a filler-containing epoxy resin 10 is formed as an insulating resin to cover both an opening end 11 of the conductive layer 4 and adjacent parts thereto for sealing an opening end 11 of the conductive layer 4 from oxygen thereby preventing oxygen from entering through the filler-containing epoxy resin 10 into the opening end 11 of the conductive layer 4. The prevention of entry of oxygen into the opening end 11 of the conductive layer 4 results prevention of oxidation reaction at a high temperature. This prevention of oxidation reaction prevents increases in resistivity and equivalent series resistance (ESR) of the capacitor.
As described above, various developments have now been made of the solid electrolyte capacitor using the conductive polymer as the solid electrolyte for the purposes of further improvements in electric characteristics and reliability of the capacitor. Further, prior to chemical polymerization or electrolyte polymerization, the silane coupling agent is used for the surface treatment of the dielectric oxide coating film for improvement in adhesiveness between the dielectric oxide coating film and the conductive polymer layer. Notwithstanding, the prior art had taken no account of improvement in adhesiveness, during polymerization, between the conductive polymer compound layer and the dielectric oxide coating film on the surface of the pore-portion. For those reasons, the solid electrolyte capacitor using the conventional is engaged with the following problems. The conductive polymer layer is likely to be peeled from the surface of the pore-portion during the formation of the conductive polymer layer on the surface of the pore-portion. This peeling, of the conductive polymer layer from the surface of the pore-portion results in an increase in leakage of current and a deterioration in equivalent series resistance, whereby the reliability of the capacitor is thus lost.
FIGS. 2A through 2B are fragmentary cross sectional elevation views illustrative of a conventional method of forming conductive polymer layers over a porous surface.
With reference to FIG. 2A, an dielectric oxide coating film 2 is formed over surfaces of a pore-inside portion 21 of a tantalum porous pellet 1 and a pore-outside portion 22. A silane coupling agent layer 3 is laminated on the dielectric oxide coating film 2.
With reference to FIG. 2B, a first conductive polymer layer 4A is formed on the silane coupling agent layer 3, however, in the pore-inside portion 21 by dipping several times of the pellet 1 into a polymerization reaction solution, wherein the silane coupling agent layer 3 except in the pore-inside portion 21 is etched by the polymerization reaction solution.
With reference to FIG. 2C, a second conductive polymer layer 4B is formed over the pore-outside portion 22 but on the dielectric oxide coating film 2. Namely, the second conductive polymer layer 4B is laminated directly on the dielectric oxide coating film 2 over the pore-outside portion 22 without, however, intervening the silane coupling agent layer 3, for which reason no intervention of the silane coupling agent layer 3 between the dielectric oxide coating film 2 allows peeling of the second conductive polymer layer 4B.
In the above circumstances, it had been required to develop a novel solid electrolyte capacitor using a conductive polymer layer as a solid electrolyte which is free from peeling from an outer surface outside a pore of a porous surface for improvement in reliability of the capacitor and a method of forming the same.